This invention pertains to improving the tunability of NMR and MRI coils, particularly in double-resonance multinuclear experiments on large samples at high fields, by means of novel paralleling and rf feed arrangement on saddle coils and the like for use on cylindrical surfaces aligned with Bo. Related NMR coils are described by Zens in U.S. Pat. No. 4,398,149, Hill and Zens in U.S. Pat. No. 4,517,516, and Doty in U.S. Pat. No. 4,641,098, and again by Doty in a copending application.
Center-fed Alderman-Grant resonators (J. Magn. Reson. 36, 1979) are disclosed by Nishihara and Yoda in U.S. Pat. No. 4,837,515. Hayes uses center feeding in his low-pass birdcage in U.S. Pat. No. 4,694,255. Unsegmented saddle coils (saddle coils of at least one full turn without a segmenting capacitor) have always been fed from one end for the simple reason that this reduces lead inductance and lead resistance. Also, notwithstanding U.S. Pat. No. 5,192,911 by Hill and Cummings, for at least the past twelve years, it has been standard practice to fully shield the leads and tuning elements from the sample with internal rf shields.
The Zens U.S. Pat. No. 4,398,149 illustrates the traditional methods of connecting spiral windings (referred to as semicoils) on opposite sides of a cylinder in series. More recently, as magnet technology has progressed to higher fields, it has become common to connect 2-turn semicoils in parallel, thereby achieving the higher B.sub.1 homogeneity that is possible with the conventional 4-turn saddle coil but at the lower inductance of the 2-turn saddle coil.
Prior art inter-connections of semi-coils, whether series or parallel, have always been made beyond the axial end of the high-B.sub.1 sample region. There are a number of convincing reasons for this tradition: (1) it reduces disturbances in B.sub.0 homogeneity within the sample region caused by the unavoidable use of slightly magnetic materials (such as copper, silver, and dielectrics); (2) it reduces disturbances in B.sub.1 homogeneity within the sample region from both induced and driven currents in the jumpers and leads; (3) it permits reduced parasitic lead capacitance, inductance, and resistance. However, there are a number of applications where it is desirable to sacrifice all of the above advantages for a single compelling reason: to be able to provide double-resonance multi-nuclear capability in large samples at high fields with minimal tuning complications and loss in efficiency from spurious resonances.
The NMR spectroscopist often finds it necessary to observe a wide variety of nuclides, especially. .sup.13 C, .sup.1 H, .sup.19 F, .sup.27 Al, .sup.29 Si, .sup.23 Na, .sup.2 H, and .sup.15 N in the study of commercially and scientifically important chemicals, and considerable interest is developing in multi-nuclear localized MR spectroscopy. Often it is important to simultaneously decouple dipolar effects of .sup.1 H; and inverse experiments, in which the effects of decoupling a low-gamma nuclide are observed in the .sup.1 H spectra, have become extremely powerful and popular. Multi-nuclear double-tuning is readily achieved in prior art designs with sample diameters up to 12 mm at fields up to 9.4 T with multi-turn saddle coils having inductance typically in the range of 30 to 70 nH. Multi-nuclear triple-resonance is available for 5-mm samples at fields up to 17.6 T (750 MHZ). A copending application discloses coil geometries suitable for double-resonance multinuclear tuning for large samples at high fields with improved B.sub.1 homogeneity.
We have discovered that paralleled semi-coils are susceptible to twin-line (parallel transmission wires) resonance modes, in which each entire semi-coil on each side behaves as if it were a solid planar conductor (rather than a spiral or parallel spirals, for example) at frequencies that may cause serious tuning difficulties. That is, at sufficiently high frequencies, the currents in all axially aligned portions of the spiral are in phase with respect to the z axis rather than with respect to the low-frequency (LF) spiral path (curl of transverse B.sub.1). As a result of the relatively large capacitance to the internal floating rt cylindrical shield and perhaps to another orthogonal coil and closely spaced external shield, the lowest frequency twin-line semi-coil mode is generally the differential mode. For the conventional configuration with the leads from each semi-coil oriented toward one end of the coil form, the if voltages in this mode on ea Of each the leads is zero somewhere near the point where they are paralleled, the voltage at the remote end of one semi-coil is a maximum with phase .phi. and the voltage at the remote end of the opposite semi-coil is a maximum with phase .phi.+.pi.. The B.sub.1 from the differential twin-line mode is generally orthogonal to the z axis and to the LF B.sub.1 axis.
The twin-line modes of paralleled semi-coils are not known to cause problems in single-tuned multinuclear applications, as one is normally limited in these cases to operation at frequencies below the fundamental self-resonance of the complete coil, which is normally much lower than the lowest twin-line mode. However, when two orthogonal saddle coils are used in double-tuned multinuclear situations such as .sup.1 H-X.sub.1 the differential twin-line mode of the multi-X (low-gamma) coil could easily be very near the .sup.1 H frequency. It is usually not difficult to shift this twin-line mode downward a substantial amount by adding capacitance between the remote ends of the problematic saddle coil, but since the B.sub.1 from the twin-line mode is approximately collinear with the B.sub.1 from the normally behaving orthogonal .sup.1 H (proton) coil, these two modes are strongly coupled and the proton Q is severely degraded unless this spurious resonance is well above the proton frequency.
The instant invention provides a simple method of approximately doubling the frequency of the differential twin-line line mode of paralleled semi-coils. This nearly doubles the limiting frequency-diameter product for which efficient multinuclear double resonance is practical.